feat: per-commit branch label and --project filter

[ashishxcode]

What

Two related additions to make worklogs branch-aware and scopeable to a single project.

Per-commit branch label

• model.Item gains a Branch field.
• collect: git log now uses --all --source and captures %S, the ref each commit was reached from; shortRef() strips refs/heads/, refs/remotes/<remote>/, and refs/tags/ prefixes to a clean branch name.
• render: the commit markdown line now leads with the branch (linked to the commit) and keeps the repo as context, dropping the bare @hash. JSON output gains a branch field.
• tui picker: new branch column, and branch is included in the / filter haystack.

Markdown example:

- feat: add agency customization fields  _([feat/agency-customization/02-06](…/commit/2d4b0ca) · saas-super-admin)_

--project filter

• New --project <name> flag narrows the resolved repos to a single one by directory name (case-insensitive), erroring clearly if no configured repo matches.

commit-chronicle --project saas-super-admin --since "7 days ago"

Notes

• Branch is resolved from git's source ref, so after a feature branch is merged/squashed into an integration branch, those commits may report the integration branch (e.g. development) rather than the original feature branch. Follow-up could prefer feature branches when a commit exists on both.

Test plan

• go build ./..., go vet ./..., go test ./... all pass.
• Verified end-to-end (markdown + json + picker) against real repos: branch resolves per commit, --project filters correctly, unknown project name errors cleanly.